Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 1 l-Monotone Convex polygons are easy to triangulate. Unfortunately the partition into convex parts is just as difficult as the triangulation. l-monotone l P A simple polygon is called monotone w.r.t. a line l if for any line l´ perpendicular to l the intersection of the polygon with l´ is connected (y-monotone, if l = y-Axis). Observation: P is y-monotone.

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 2 Two steps for triangulation 1. Divide P into y-monotone parts P 1,...,P k 2. Triangulate P 1,...,P k

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 3 Split and Merge Vertices = start vertex = end vertex = regular vertex = split vertex = merge vertex

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 4 Five Types of Vertices = start vertex = end vertex = regular vertex = split vertex = merge vertex

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 5 Theorem A simple polygon with n vertices can be partitioned into y-monotone polygons in O(n log n) time with an algorithm that uses O(n) storage.

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 6 Triangulation of y-monotone Polygon Idea: Fan so long build to convexity hurts alternation from right and left side not yet triangulated Implementation: Scan-line uses stack as data structure Case 1: Page overflowsCase 2: resembles page popped pushed

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 7 Example a b c d e fh i jk l m n o p q r s t u v w x Batches : ba c : ba  ca d : ca  dc e : dc  ed g

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 8 a b c d e f g h i j

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 9 Implementation 1.S.push(u 1 ), S.push(u 2 ) 2. for j = 3,...,n-1 3. if (side(u j )  side(S.top)) 4. while (S   ) v = S.pop, diag(u j,v) 5. S.push(u j-1 ) 6. S.push(u j ) 7. else 8. while (diag(S.top, u j ) in P) 9. diag(S.top, u j ) 10. S.pop 11. S.push(last) 12. S.push(u j ) ujuj ujuj Theorem: time O(n) Proof: number of pops < number of pushes

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 10 Theorem Theorem: A strictly y-monotone polygon with n vertices can be triangulated in O(n) time. Theorem: A simple polygon with n vertices can be triangulated in O(n log n) with an algorithm that uses O(n) storage. Theorem: A planar subdivision with n vertices in total can be triangulated in O(n log n) time with an algorithm that uses O(n) storage.

Lecture 3: Polygon Triangulation Computational Geometry Prof. Dr. Th. Ottmann 11 Computational Geometry Algorithms Library Kernel 2D/3D point, vector, direction, segment, ray, line, dD point, triangle, bounding box,iso-rectangle, circle, plane, tetrahedron, predicates, affine transformations, intersection and distance calculation Basic Library half edge data structure, topological map, planar map, polyhedron, Boolean operations on polygons, planar map overlay, triangulation, Delauney triangulation, 2D/3D convex hull, and 2D extreme points, smallest enclosing circle/sphere and ellipse, maximum inscribed k-gon, and other optimizations, range tree, segment tree, kD tree